Safe-arm mechanism for explosive trains



Sept. 19, 1967 G. A. NODDIN SAFE-ARM MECHANISM FOR EXPLOSIVE TRAINS Filed July 6,1965 I INVENTOR. G. A. N o d d in United States Patent Ofilice 3,34Z,l3l Patented Sept. 19, 1967 3,342,131 SAFE-ARM MECHANISM FOR EXPLOSIVE TRAINS George A. Noddin, Mantua, Ni, assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Filed July 6, 1965, Ser. No. 469,937 7 Claims. (Cl. 10222) ABSTRACT OF THE DISCLOSURE Detonating fuse formed into a loop having a narrow open neck at its base will not propagate a detonation stimulus when in a gaseous medium such as air, but will propagate such a stimulus when immersed in a liquid medium such as water. Accordingly, detonating fuse of this configuration is a safe-arm mechanism for explosive trains designed, e.g., for underwater use.

For activities in which the detonation of explosive charges below the surface of water is desired, such as in seismic exploration, underwater signaling, echo-ranging for detection purposes, and destruction of undersea craft, there is a need for a reliable and accurate explosive train which can be armed, i.e., made capable of propagating an initiation impulse, when submerged, yet will be safe, i.e., disarmed by virtue of being incapable of propagating a detonation stimulus, under conditions of storage or removal from the water or other liquid.

Commonly, mechanisms for precluding accidental actuation of all components of explosive trains comprising a donor charge and receptor charge in explosive assemblies intended for underwater use depend upon physical separation of the components by maintaining these elements out of line or separated by a gap sufiicient to preclude actuation of the receptor charge, should accidental actuation of the donor charge occur. To arm such an explosive train, mechanisms are provided to move the components into position, i.e., nearer to one another or into line, so that continuous detonation of the explosive train will occur upon actuation of the donor charge. This type of arming mechanism generally requires the use of complex mechanical systems which, in order to function properly, must be in prescribed alignment and adjusted to move the elements smoothly from the safe to the arm positions. However, the requisite alignment and adjustment are difiicult to insure since the mechanisms are susceptible to damage during handling and launching of the explosive assembly containing the arming system. Further, in such mechanisms, the arming often is irreversible, thus once armed, such assemblies must be fired.

Recently, an arming system has been proposed wherein the donor charge and the receptor charge in an explosive train are separated by a fixed distance sufiicient to preclude actuation of the receptor charge by the donor charge when the two are in air but allow propagation of a detonation impulse from the donor to the receptor charge when the two are in a liquid, i.e., in water. The train is armed by changing the medium in the space separating the donor and receptor charges, i.e., the train is armed by replacing the air, normally occupying this space in the assembly before the train is armed, by a liquid, i.e., water, by immersing the train in the liquid or by pumping liquid into the space. This system, which is described in US. 3,065,- 694 is based on the theory that shock waves are conducted more etficiently by a liquid, e.g., water, than by a gaseous medium, e.g., air. Although such a system is attractive in that it would minimize or eliminate the need for complex arming mechanisms, the theory upon which the system is based does not prove valid or practicable for commercially available explosive charges. Rather, it has been determined experimentally, using commercially available explosive charges and accessories, that a detonation impulse is not propagated as readily between spaced apart donor and receptor charge when the train is in a liquid, i.e., water, as when the train is in air. For example, when a fii-inch long piece of detonating cord containing a central core of lead azide at a loading of 34 grains per foot is used in a thin-bottomed tubular aluminum cap shell as the donor charge, and the receptor charge is 4 grains of PETN pressed into a thin-bottomed tubular aluminum cap shell (0.240 inch bottom thickness), a spacing of at least inch must be provided to preclude actuation of the receptor charge by the donor charge when the two are in air. However, when Water occupies the space between the charges, the detonation impulse cannot be propagated from the donor charge to the receptor charge over this distance. In fact, when the space is occupied by Water, the spacing must be inch or less to allow propagation from the donor to the receptor charge. Thus, an arming mechanism in which the donor charge and receptor charges of an explosive train are separated by a distance sufficient to preclude propagation of a detonation impulse in air would still preclude propagation when the charges are in water, i.e., the train would not be armed in water.

Accordingly, a need still exists for an arming system for underwater use in which the safe-arm mechanism is not dependent upon the interaction of mechanical parts but is positive and reliable in functioning. Such a system should be reliably operational at both shallow and great depths and should be easily incorporated into the explosive train.

In accordance with this invention, it has been found that certain detonating fuses having a core of cap-sensitive, high-velocity detonating explosive encased in a flexible plastic and/or textile sheath will not propagate a detonation stimulus in a gaseous medium such as air but will propagate such a stimulus in a liquid medium such as water or oil if a segment of the fuse is in the form of a loop with a narrow open neck at its base. The neck of this safe-arm configuration is provided by bringing two separate sections along the length of fuse side-by-side so that they are closely adjacent to but spaced from each other, and the loop is simultaneously formed by the integral portion of fuse connecting these adjacent sections. The space between the adjacent sections forming the neck, i.e., the Width of the opening through the neck, is such that in a gaseous medium the brisance of detonation in one of these sections will rupture and scatter the adjacent section without sympathetically actuating it, and in a liquid medium the detonation stimulus will be propagated without hiatus from one section through the loop to the adjacent section. The length of cord in the loop connecting the adjacent sections is such that in the gaseous medium, the detonation stimulus will not have sufiicient time to pass through the loop before rupture and scattering of one such section by the other is complete.

The above loop and neck configuration is particularly effective safe-arm configuration for fuses wherein the core of cap-sensitive, high-velocity detonating explosive is at a loading of about from 40 to grains/ foot. Accordingly, the present invention provides a safe-arm mechanism which comprises a segment of detonating fuse in the form of a loop with a narrow open neck at its base, said fuse comprising a core of cap-sensitive, high-velocity detonating explosive at a loading of about from 40 to 85 grains/ foot encased in a flexible textile and/or solid plastic sheath, the width of the opening through the neck and the length of fuse in the loop being such that said segment will propagate a detonation stimulus when the neck is immersed in a liquid medium but will not propagate such a 3 stimulus when the neck is surrounded by a gaseous medium.

This invention also provides an arming system wherein an exposed safe-arm mechanism, as described above, connects a donor charge (e.g., an initiator) to a receptor charge (e.g., a base, booster or primer charge) which is spaced from the donor charge at least a distance sufficient to preclude sympathetic actuation of the receptor charge by the donor charge.

The term exposed is used herein with reference to the safe-arm mechanism of this invention todenote that substantially all of at least the adjacent sections of fuse forming the open neck at the base of the loop are neither enclosed nor confined in such a manner that they are in- Capable of being immersed in, and thus surrounded by, the arming liquid medium.

In order to describe the invention in greater detail, reference is now made to the accompanying drawing, which is a view of an arming system and safe-arm mechanism of this invention.

In the drawing D designates the donor charge and R designates receptor charge of the arming system. These charges are separated so that donor charge D will not sympathetically actuate charge R in air and thus water. Extending between charges D and R is a length of explosive detonating fuse F comprising a core 1 of highvelocity, cap-sensitive detonating explosive in a flexible sheath 2, e.g., of extruded nylon or braided nylon filament. Segment S of the fuse is provided as a safe-arm mechanism and consists of a loop L having at its base a narrow open neck defined by adjacent sections A-A' and BB of the fuse. This loop and neck configuration is maintained by threading the sections AA and B-B through apertures 3 and 4- which extend transversely through hollow tubular retainers 5. Access of the arming liquid medium to the neck of the fuse is provided by the bore in each of these tubular retainers.

In the above-described arming system, the width of the opening between sections AA' and B-B' and the length of fuse in loop L is such that in a gaseous medium, when the detonation stimulus from donor charge D detonates section A-A', brisance of this sections detonation will rupture and scatter section B-B' before the detonation stimulus has had sufiicient time to travel the length of the loop. Thus, passage of the detonation stimulus through section BB' and on to receptor charge R is precluded. However, when sections A-A' and B-B' are in a liquid medium, e.g., oil or water, BB' is not destroyed by the detonation stimulus in A-A', but is propagated without hiatus through the loop, section BB' and on to charge R. The requisite length of loop L depends primarily on the detonation velocity of the fuse, longer loops being used with higher velocity fuses, and the spacing between sections A-A and B-B' depends principally on the fuses core loading and the ratio of that loading to the thickness of a given sheathing material in which the core is encased, larger core loadings and ratios requiring greater minimum spacings between A-A and B-B'.

The detonating fuse employed in the safe-arm mechanism of this invention comprises a core of cap-sensitive, high-velocity 1200 meters/sec.) detonating explosive at a core loading of about 40 to about 85 grains per foot. Examples of such explosives are pentaerythritol tetranitrate (PETN), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), tetryl, tetranitrodibenzo 1,3a,4,6a (or 1,3a,6,6a) tetraazapentylene, TNT, lead azide, nitromannite, picryl sulfone, bis(trinitroethyl)urea and mixtures thereof. Such explosives can be used alone or in combination with one or more additives, e.g., conventional graining agents and binders therefor provided the core loading exclusive of such additives is in the above range. Preferred core compositions for many underwater applications from the viewpoint of water-resistance, stability, and flexibility over a wide range of conditions of temperature and pressure, are of the types described in US. Patent 2,992,087, which comprise at least 44 weight percent of superfine PETN in combination with a binder of nitrocellulose and a trialkyl ester of Z-acetoxy-1,2,3-propanetricarboxylic acid (ATC); in US. Patent 2,999,743 comprising 92.5 to 70 weight percent of high explosive and 7.5 to 30 percent of a binder comprising 25 to 75 percent of an organic rubber, e.g., butyl rubber, and 75 to 25 percent of a thermoplastic terpene hydrocarbon resin; and those described in copending patent application Serial No. 437,555, filed March 5, 1965, now Patent No. 3,311,513, which comprise RDX or HMX in a plasticized nitrocellulose binder.

The core of explosive composition will be contained in a protective sheath of one or more layers of braided, woven or knit textile; of solid plastic, e.g., in the form of tape or an extruded tube; or of a combination of such materials. Suitable textile compositions include cotton,

,rayon, and nylon, the latter being preferred because of its high strength at low deniers. Nylon, polyethylene, Hycar rubber, and polyvinyl chloride are exemplary plastics which may be used alone or in combination with one or more layers of textile to form a suitable flexible sheath. Where textile sheathing is employed, the water and abrasion resistance of the fuse may be improved by impregnating the textile with materials such as asphalt or wax. Also, adhesives may be employed to prevent slippage of the core within the sheath or to fasten separate layers together.

The sheathing thickness which will permit shattering of the sheath in a gaseous medium but preclude this rupturing or shattering in a liquid will, of course, depend upon the explosive strength and brisance of the explosive composition in the core, as well as the strength and ductility of the sheathing. As a rule, however, the ratio of core loading (grains/ foot) to sheathing thickness (inches) will be within the range of about from 1000 to 8000, the higher ratios being permissible with braided high-strength textile sheaths, e.g., those prepared from nylon filament.

Commercially available fuses suitable for use in the safe-arm mechanism of this invention are plain, reinforced, and plastic reinforced Primacord, having a core of PETN or RDX at a loading of about from 40 to grains/foot. These fuses comprise a raw core of pressed, pulverent PETN or RDX encased in an inner sheath, i.e., a sock of braided cotton covered with a multilayer strengthening sheath. For plain Primacord this multilayered sheath is comprised of asphalt, a cotton tape, braided cotton countering, and a wax finish, reinforced Primacord differs from plain in that the braided cotton countering is double the thickness of that on plain. For plastic reinforced Primacord the inner cotton sheath or sock is covered by a multilayered sheath of asphalt, high tenacity rayon and a plastic such as polyethylene, nylon, polyvinyl chloride resin, or a Hycar rubber. These fuses have a ratio of core loading in grains/foot to sheathing thickness in inches of about 1100 to 2000. Other commercially available fuses suitable for use in the safe-arm mechanism are fuses having a core of Detaflex C (the composition of US. 2,992,087 containing, by weight, about 63% PETN, 28% ATC, 8% nitrocellulose and 1% dye) at a PETN loading of from 45 to 80 grains/foot encased in a single layer of braided 420 to 840 denier nylon. These fuses have a ratio of core loading in grains/foot to sheathing thickness in inches of about from 5000 to 8000.

As noted above, the width of the opening through the neck of the safe-arm mechanism (i.e., the horizontal distance between A-A' and BB' in the drawing) must be such that the brisance of detonation in section AA will rupture and scatter section B-B without initiating B-B' when these sections are surrounded by a gaseous medium. This width must, however, be great enough that B-B' will not be thus damaged when the neck is immersedin a liquid medium. Also, the length (i.e., circumference) of the loop connecting sections A-A' and B-B' must be sufficient to prevent the detonation stimulus within the fuse from reaching B-B' before it is completely ruptured and scattered. It is seen, therefore, that the optimum width of the opening through the neck and the optimum length of the loop will depend on the characteristics of the particular fuse employed. For example, using the above Detaflex C fuse wherein the core loading of PETN is 47 grains/foot and the sheath is a single layer of braided 420 denier nylon, openings and 7 of an inch wide and a loop of at least 4 inches is satisfactory; using 77 grain/foot-Detaflex C fuse having a sheath of braided 840 denier nylon, openings /8 to inch wide and a loop length of 8 or more inches will usually be required; and with the above Primacords at a PETN loading of 50 grains/foot best results are obtained with openings of and inch and a loop of about 8 or more inches long. In general, however, the width of the opening through the neck will be from to about /2 inch, and the minimum loop length will be about from 4 to 8 inches for core loadings of about from 40 to 85 grains/ foot, respectively.

The shape of the loop is not critical provided it is of sufficient length. It will be of generally circular or elliptical configuration and will have a minimum diameter in excess of the width of the opening through the neck of the safearm mechanism.

The length of fuse sections AA and B-B' which define the neck should be at least about /2 inch to insure that section BB' is ruptured and scattered and thus the detonation stimulus is cut off in the gaseous medium. Lengths in excess of inch are not required but may be employed.

As shown in the drawing, retaining means are employed to maintain the requisite loop and neck configuration. Such means include clips, tubular holders as in the drawing, wire, thread, etc. which permit the arming liquid medium to contact and surround substantially all of the neck of the safe-arm mechanism. Polyethylene tubes through which the fuse has been threaded as in the drawing have been found particularly satisfactory for this purpose.

As mentioned above, the spacing between the donor and receptor charges in the arming system of this invention must be such that the donor charge will not sympathetically acuate the receptor charge. For some applications it may be desirable to have this distance sufficient to preclude sympathetic actuation of either charge by the other. The distance between these charges can be minimized by employing baflles between them. The spacing between the donor and receptor charges will normally be shown in accordance with the air gap sensitivity of the receptor charge; however, if the arming system is to function in another gaseous environment, the gap sensitivity of the receptor charge in that environment will be controlling. The spacing between charges can be maintained by mounting the charges on a suitable supporting means, e.g., of wood, netting, metal, etc. The arming system can be mounted in a housing provided it permits access of the arming liquid to at least the neck portion of the safe-arm mechanism. If desired, the housing can be provided with a reservoir for the liquid medium and a conventional two-way pump, which can be actuated by a timing mechanism, to pump the liquid to and from the chamber containing the safe-arm mechanism.

A variety of means may be used to initiate detonation in the donor charge. A pressure-actuated initiator, i.e., detonator, is particularly suitable since actuation of the donor charge will occur only in a liquid at a predetermined depth, i.e., pressure. However, actuation of the donor charge can also be initiated by an initiator having a conventional electric ignition assembly or an electric delay ignition assembly fired by a bridgewire in a loose ignition composition, a bridgewire and bead arrangement, an exploding bridgewire, or are firing system, in which case the lead wires of the electric ignition assembly extend to a source of electric current at the surface of the water and the sink rate of the explosive assembly must be known. Alternatively, the donor charge can be actuated by an electric detonator fired by a fluid (water)-actuated battery, e.g., a sea battery and a pressure-sensitive switch which will fire the detonator at a predetermined depth.

The following example illustrates specific embodiments of the invention.

Example Ten explosive trains are set up in which sheets of the extrudable explosive composition described in US. 2,992,087 that is commercially available as Detaflex C are used as the donor and receptor. The charges in each array measure 2 x 2 x inch and contain 24 grams of explosive. The charges are separated by a distance of 1 foot, suflicient to preclude sympatheic detonation of one charge by influence of the other in air and water. A commercially available E-94 blasting cap having a base charge of 2 grains of PETN is in propagating relationship to the donor charge. The two charges are connected by a length of detonating fuse having a core of Detaflex C at a PETN loading of 77 grains/foot in a sheath formed of a single layer of braided 840 denier nylon (sheathing thickness, 0.015 inch), the ratio of core loading to sheathing thickness being 5133. The length of fuse is provided with a safe-arm segment as shown in the drawing by bringing the two 2.5 inch long segments (AA and B-B') which are spaced eight inches apart along the length of the detonating fuse, side-by-side. The segments of fuse are maintained in this side-by-side relationship by passing them through aligned No. 12 drill apertures 3 and 4 in two A1- inch outer diameter, 1 inch long, 0.25 inch I.D., 0.375 in. OD. polyethylene tubes 5, the distance between openings 3 and 4 and thus between AA' and B-B' being inch. The eight inch length of cord between the two sideby-side sections forms a generally circular loop.

For comparative purposes, ten explosive trains are set up as above but with the blocks of explosive composition connected by a substantially straight length of the detonating fuse.

Five of the explosive trains of each type are initiated in air at atmospheric pressure. In the trains containing the safe-arm mechanism of this invention, the receptor charge is not actuated even though the donor charge functions properly in all five cases. In the explosive trains having the donor and receptor charges connected by a straight length of detonating fuse, a detonating impulse is propagated from the donor charge to the receptor charge without hiatus.

When five of each type explosive train are actuated in water, a detonation stimulus initiated in the donor charge i propagated, without hiatus, to the receptor charge.

As mentioned earlier, in the arming system of this invention the segment of fuse formed into the safe-arm configuration can be protected by a shell provided it will permit the arming fluid to contact and surround at least the adjacent sections of fuse which define the neck of the safe-arm mechanism. Means can be provided for regulatin the flow of arming fluid into a chamber containing the safe-arm mechanism so that the positive arming of the arming system can be regulated in accordance with the needs of a particular situation, e.g., positive arming can be delayed to insure safety or made to coincide with actuation of a hydrostatic pressure-armed initiator. The presence of a liquid such as water or oil in suflicient quantity to surround the neck of detonating fuse will cause arming of the device regardless of the depth of the liquid in which the explosive train is submerged. The simple exchange of, e.g., water for air, within the chamber eliminates the necessity of complicated mechanisms or moving parts and provides an arming system which is reliable and nonhazardous. The explosive train can be disarmed or rendered safe should its recovery in desired, simply by replacing the liquid in the chamber with a gaseous medium. It is to be understood, however, that a protective shell need not be used and the arming system may comprise simply blocks or slabs of explosive charge as the donor and receptor charges maintained at a predetermined spacing, e.g., by a support means, and connected by the detonating fuse of loop and neck configuration.

What is claimed is:

1. A safe-arm mechanism comprising a segment of detonating fuse in the form of a loop with a narrow open neck at its base and having a core of cap-sensitive, high-velocity detonating explosive at a loading of about from 40 to 85 grains/foot encased in a flexible sheath of material selected from the group consisting of textile, solid plastic, and a combination of textile and solid plastic, the width of the opening through the neck being from to about /2 inch and the minimum loop length being about from 4 to 8 inches for core loadings of about from 40 to 85 grains/foot, respectively, said width and length being such that the segment of detonating fuse will propagate a detonation stimulus when the neck is immersed in a liquid medium but will not propagate such a stimulus when the neck is surrounded by a gaseous medium.

2. A safe-arm mechanism of claim 1 wherein said detonating fuse is Primacord having a core selected from the group consisting of PETN and RDX, a core loading of about from 40 to 80 grains/foot and a ratio of core loading in grains/ foot to sheathing thickness in inches of about from 1100 to 2000.

3. A safe-arm mechanism of claim 1 wherein said detonating fuse has a core of at least about 44% PETN,

by weight, in a flexible binder encased in a braided nylon sheath, the core loading of PETN being about from 45 to 80 grains/foot and the ratio of core loading in grains/ foot to sheathing thickness in inches being about from 5000 to 8000.

4. An arming system comprising a donor charge, a receptor charge separated from the donor charge by distance 'suflicient to preclude sympathetic actuation of the receptor charge by a detonation stimulus from the donor charge, and a safe-arm mechanism connecting said charges, said safe-arm mechanism comprising a segment of detonating fuse in the form of a loop with a narrow open neck at its base and having a core of cap-sensitive, high-velocity detonating explosive at a loading of about from 40 to 85 grains/ foot encased in a flexible sheath of material selected from the group consisting of textile, solid plastic, and a combination of textile and solid plastic, the ratio of said core loading in grains/ foot to sheathing thickness in inches being aboutfrom 1000 to 8000, the width of the opening through the neck being from A to about /2 inch, and the minimum loop length being about from 4 to 8 inches for core loadings of about from 40 to 85 grains/foot, respectively, said Width and length being such that the segment of detonating fuse will propagate a detonation stimulus when the neck is immersed in a liquid medium but will not propagate such a stimulus when the neck is surrounded by a gaseous medium.

5. An arming system of claim 4 wherein the said distance between the donor and receptor charges is sutficient to preclude said sympathetic actuation in both air and water, and said gaseous medium is air and said liquid medium is water. 7, V

6. An arming system of claim 5 wherein the detonating fuse is Primacord having a core of PETN at a loading of about grains/ foot and a ratio of core loading to sheath ing thickness of about from 1100 to 2000 and wherein the width of the opening through the neck is from to inch and the length of the said loop is at least about 8 inches.

7. An arming system of claim 5 wherein said detonating fuse is a core of about 63% PETN, about 28% of a trialkyl ester of Z-acetoxy-1,2,3-propanetricarboxylic acid, and about 8% nitrocellulose encased in a sheath composed of a single layer of braided 840 denier nylon, the PETN core loading being about 77 grains/foot, and wherein the width of the opening through the neck is to 7 inch and the length of said loop is at least about 8 inches.

References Cited UNITED STATES PATENTS 2,725,820 12/ 1955 McCullough 10220 3,021,785 2/1962 Hradel et al 102--70 X 3,169,481 2/1965 Stresau et al 10227 FOREIGN PATENTS 486,424 11/1953 Italy.

BENJAMIN A. BORCHELT, Primary Examiner.

G. H. GLANZMAN, Assistant Examiner. 

1. A SAFE-ARM MECHANISM COMPRISING A SEGMENT OF DETONATING FUSE IN THE FORM OF A LOOP WITH A NARROW OPEN NECK AT ITS BASE AND HAVING A CORE OF CAP-SENSITIVE, HIGH-VELOCITY DETONATING EXPLOSIVE AT A LOADING OF ABOUT FROM 40 TO 85 GRAINS/FOOT ENCASED IN A FLEXIBLE SHEATH OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF TEXTILE, SOLID PLASTIC, AND A COMBINATION OF TEXTILE AND SOLID PLASTIC, THE WIDTH OF THE OPENING THROUGH THE NECK BEING FROM 1/16 TO ABOUT 1/2 INCH AND THE MINIMUM LOOP LENGTH BEING ABOUT FROM 4 TO 8 INCHES FOR CORE LOADINGS OF ABOUT FROM 40 TO 85 GRAINS/FOOT, RESPECTIVELY, SAID WIDTH AND LENGTH BEING SUCH THAT THE SEGMENT OF DETONATING FUSE WILL PROPAGATE A DETONATION STIMULUS WHEN THE NECK IS IMMERSED IN A LIQUID MEDIUM BUT WILL NOT PROPAGATE SUCH A STIMULUS WHEN THE NECK IS SURROUNDED BY A GASEOUS MEDIUM. 